In the manufacture of pre-molds for optical fibers or for semiconductor engineering, intermediate products are frequently used, the silicon dioxide mold body being present as a so-called soot body. For sintering, the mold body is fed into the heating zone of a zone melting furnace in which the mold body is heated to its softening temperature; thus, the transition is initiated. Because of the high quality demands, above all with applications in communication and semiconductor engineering, sintering is done in a gas-tight chamber to reliably avoid any contaminations.
Usually, axially traveling columns are used as hoisting appliances to which the mold body is fixed upright or hanging. Such a hanging arrangement is known for example from EP-A 0,416,614, EP-A 0,529,694, and EP-A 0,547,560, in which the porous mold body is fixed hanging to a vertically or rotatingly mobile column and can thus be fed to the heating zone within the chamber which is filled with a low-reaction gas. Similarly, in U.S. Pat. No. 5,032,079 the porous mold body is fixed hanging to a vertically or rotatingly mobile column within the chamber which is filled with an inert gas, for example, nitrogen or helium.
Comparably, WO 93/23,341 A1 provides for a device for sintering with a chamber operable at ambient pressure or in a vacuum in which the mold body is arranged hanging at a vertically mobile column rotatable around its axis.
To seal the column off from the interior of the chamber, a state-of-the-art seal is provided at the duct which guarantees reliable sealing while being insensitive to the high temperatures occurring during the sintering process. With the described state of the-art, it is a disadvantage that in practice the seal is frequently subject to considerable wear, causing quick deterioration of the sealing effect, always resulting in reduced quality of the mold body after sintering.
Here, it is especially problematic that small quantities of Cl2 are released during sintering which after opening the chamber to remove the mold body react on the ambient humidity to form HCl, which is aggressive to the surface of the column. When the column is moved, this increased roughness of the surface damages the seal. Accordingly, the components of the hoisting appliance require much maintenance and care and must be treated or replaced regularly, resulting in reduced availability of the appliance. If a damage to the seal is not recognized promptly, the sealing oil present in the seal can penetrate into the inside of the chamber where it will evaporate under the high temperatures, causing contaminations in the chamber which will be detectable in the final product.
Furthermore, deposits, consisting above all of SiO2, occur on the surface of the section of the column extending into the interior of the chamber, which have to be removed regularly to avoid any damages and thus leakages of the duct.
The above mentioned problems occur more frequently with a hanging arrangement in which the duct is above the mold body and in which under the influence of gravity, the sealing oil can easily penetrate into the inside of the chamber; such problems cannot be avoided even with a standing arrangement of the hoisting column, as because of the relative axial movement between the hoisting column and the duct, inadmissible quantities of sealing oil penetrate into the inside of the chamber even through minor damages to the seal and will subsequently be detectable in the mold body.